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Our article1 on the expansion of gas bubbles by xenon and nitrous oxide investigated how air bubbles of various dimensions in aqueous solution would expand when suddenly exposed to solutions containing certain gas mixtures (particularly mixtures containing xenon). The motivation behind this work was simple: Would air bubbles that were entrained while on cardiopulmonary bypass during cardiac surgery expand to a worrying extent if xenon were used during the procedure, hence potentially exacerbating damage caused by air emboli? Xenon has been proposed for use as a neuroprotectant,2 and it might be beneficial in reducing the cognitive deficits that are known to occur during cardiopulmonary bypass.3 However, if entrained gas bubbles expanded greatly, xenon may do more harm than good.

Indeed, Dr. Eckmann and his colleagues have suggested exactly that,4 based on theoretical calculations that concluded that small gas bubbles would expand rapidly and indefinitely if they were trapped in fine blood vessels. (We fully understand that the model assumes that the bubbles are constrained by the size of the capillaries.) For example, their calculations suggest that a 50-nl bubble of oxygen exposed to 70% xenon–30% oxygen would grow to 250 nl in approximately 20 min with an ever-increasing rate of growth. Because we thought that these predictions were implausible, and because there were a large number of variables that had to be estimated, we conducted our experiments, which were designed to measure bubble growth directly under a well-defined set of conditions. We studied the expansion of both air and oxygen bubbles, and the results were similar; our data show bubble expansions of the order of 10% in diameter and 30% in volume under conditions likely to be encountered during cardiopulmonary bypass. We concluded that this is unlikely to represent a significant clinical problem.

We disagree with Dr. Eckmann’s claim that his calculations predict similar expansions to those we observed. Apart from the extent of the volume expansions that were predicted,4 their most striking aspect was the ever-increasing rates of expansion that seemed to predict unlimited bubble growth. In contrast, we observed limited bubble growth with volumes tending toward finite equilibrium values. Even making allowances for the differences between the model and the gas compositions, we believe our experimental observations probably better reflects reality than the theoretical calculations that Dr. Eckmann has published. Furthermore, in our recently published feasibility and tolerability clinical study involving exposure of cardiac surgical patients to xenon while on cardiopulmonary bypass, there was no increase in the number of observed emboli, which would have occurred had the bubble size of gas emboli increased significantly.5